Java转Kotlin

Java 转 Kotlin

打印日志

• Java

System.out.println("sss");

• kotlin

println("sss")

---

常量与变量

• Java

String name = "sss";
final String name = "sss";

• Kotlin

var name = "sss"
val name = "sss"

---

null说明

• Java

String otherName;
otherName = null;

• Kotlin

var otherName :String?
otherName = null

---

空判断

• Java

if(text != null){
  int length = text.length();
}

• Kotlin

text?.let{
  val length = text.length
}
//or simply
val length = text?.length

---

字符串拼接

• Java

String firstName = "aaa";
String lastName = "bbb";
String message = "My Name is: " + firstName + lastName;

• Kotlin

val firstName = "aaa"
val lastName = "bbb"
val message = "My name is: $firstName $lastName"

---

换行

• Java

String text = "First Line
" + 
              "Second Line
" + 
              "Third Line";

• Kotlin

val text = """
          |First Line
          |Second Line
          |Third Line
          """.trimMargin()

---

三元表达式

• Java

String text = x > 5 ? "x > 5" : "x <= 5";

• Kotlin

val text = if(x > 5)
           "x > 5"
           else "x <= 5"

---

操作符

• Java

final int andResult = a & b;
final int orResult = a | b;
final int xorResult = a ^ b;
final int rightShift = a >> 2;
final int leftShift = a << 2;
final int unsignedRightShift = a >>> 2;

• Kotlin

val andResult = a and b
val orResult = a or b
val xorResult = a xor b
val rightShift = a shr b
val leftShift = a shl b
val unsignedRightShift = a ushr 2

---

类型判断和转换（声明式）

• Java

if(object instanceof Car){
  Car car = (Car)object;
}

• Kotlin

if(object is Car){
  var car = object as Car
}

---

类型判断和转换（隐式）

• Java

if(object instanceof Car){
  Car car = (Car)object;
}

• Kotlin

if(object is Car){
  var car = object
}

---

多重条件

• Java

if(score >= 0 && score <= 300){}

• Kotlin

if(score in 0..300){}

---

更灵活的case语句

• Java

int score = // some score
String grade;
switch(score){
  case 10:
  case 9:
    grade = "Excellent";
    break;
  case 8:
  case 7:
  case 6:
    grade = "Good";
    break;
  case 5:
  case 4:
    grade = "Ok";
    break;
  case 3:
  case 2:
  case 1:
    grade = "Fail";
    break;
  default:
    grade = "Fail";
}

• Kotlin

var score = // some score
var grade = when(score){
  9,10 -> "Excellent"
  in 6..8 -> "Good"
  4,5 -> "OK"
  in 1..3 -> "Fail"
}

---

for 循环

• Java

for(int i = 1; i <= 10; i++){}
for(int i = 1; i < 10; i++){}
for(int i = 10; i >= 0; i--){}
for(int i = 10; i <= 10; i += 2){}
for(int i = 10; i >= 10; i-= 2){}
for(String item : collection){}
for(Map.Entry<String, String> entry : map.entrySet()){}

• Kotlin

for(i in 1..10){}
for(i in 1 until 10){}
for(i in 10 downTo 0){}
for(i in 1..10 step 2){}
for(i in 10 downTo 0 step 2){}
for(item in collection){}
for((key, value) in map){}

---

更方便的集合操作

• Java

final List<Integer> listOfNumber = Arrays.asList(1,2,3,4);

final Map<Integer, String> keyValue = new HashMap<Integer, String>();
map.put(1, "aaa");
map.put(2, "bbb");
map.put(3, "ccc");

// Java 9
final List<Integer> listOfNumber = List.of(1,2,3,4);

final Map<Integer, String> keyValue = Map.of(1, "aaa"
                                             2, "bbb"
                                             3, "ccc");

• Kotlin

val listOfNumber = listOf(1,2,3,4)
val keyValue = mapOf(1 to "aaa"
                     2 to "bbb"
                     3 to "ccc")

---

遍历

• Java

// Java 7 and below
for(Car car : cars){
  System.out.println(car.speed);
}

for(Car car : cars){
  if(car.speed > 100){
    System.out.println(car.speed);
  }
}

// Java 8+
cars.forEach(car -> System.out.println(car.speed));

cars.stream().filter(car -> car.speed > 100).forEach(car -> System.out.println(car.speed));

• Kotlin

cars.forEach{
  println(it.speed)
}

cars.filterP{it.speed > 100}
       .forEach{println(it.speed)}

---

方法定义

• Java

void doSomething(){
  //logic here
}

void doSomething(int...numbers){
  //logic here
}

• Kotlin

fun doSomething(){
  //logic here
}

fun doSomething(vararg numbers: Int){
  //logic here
}

---

带返回值的方法

• Java

int getScore(){
  //logic here
  return score;
}

• Kotlin

fun getScore(): Int{
  // logic here
  return score
}

// as a single-expression function

fun getScore(): Int = score;

---

无结束符号

• Java

int getScore(int value){
  //logic here
  return 2 * value;
}

• Kotlin

fun getScore(value: Int): Int{
  // logic here
  return 2 * value;
}

fun getScore(value: Int): Int = 2 * value

---

constructor构造器

• Java

public class Utils{
  private Utils(){
    // This utility class is not publicly instantiable
  }
  
  public static int getScore(int value){
    return 2 * value;
  }
}

• Kotlin

class Utils private constructor(){
  companion object{
    fun getScore(value: Int): Int{
      return 2 * value
    }
  }
}

// another way
object Utils{
  fun getScore(value: Int): Int{
    return 2 * value
  }
}

---

Get Set构造器

• Java

public String name;
private int age;

public Developer(String name, int age){
  this.name = name;
  this.age = age;
}

public String getName(){
  return name;
}

public void setName(String name){
  this.name = name;
}

public int getAge(){
  return age;
}

public void setAge(int age){
  this.age = age;
}

@Override
public boolean equals(Object o){
  if(this == o) return true;
  if(o== null || getClass() != o.getClass()) return false;
  
  Developer developer = (Developer)o;
  
  if(age != developer.age) return false;
  return name != null ? name.equals(developer.name) : developer.name == null;
}

@Override
public int hashCode(){
  int result = name != null ? name.hashCode() : 0;
  result = 31 * result + age;
  return result;
}

@Override
public String toString(){
  return "Developer{" +
          "name='" + name + '" +
          ", age = " + age +
          '}';
  }
}

• Kotlin

data class Developer(val name: String, val age: Int)

---

原型扩展

• Java

public class Utils{
  private Utils(){
    // This utility class is not publicly instantiable
  }
  
  public static int triple(int value){
    return 3 * value;
  }
}

int result = Utils.triple(3);

• Kotlin

fun Int.triple(): Int{
  return this * 3
}

val result = 3.triple()

• Java

public enum Direction{
  NORTH(1),
  SOUTH(2),
  WEST(3),
  EAST(4);
  
  int direction;
  
  Direction(int direction){
    this.direction = direction;
  }
  
  public int getDirection(){
    return direction;
  }
}

• Kotlin

enum class Direction(val direction: Int){
  NORTH(1),
  SOUTH(2),
  WEST(3),
  EAST(4);
}

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原文链接：https://github.com/MindorksOpenSource/from-java-to-kotlin/blob/master/README-ZH.md